I am finishing my basement of a new built home. The builder has already insulated and dry walled the walls. I am ready to move forward on the project, but should the basement bathroom framing and ceiling be done before the floor is completed or should the flooring come first?
When adding new walls for things like bathrooms I will usually put the walls up first on the deck or slab this way if later you have a water leak and the underlayment has started to swell it is not trapped by the walls it makes repairs easier it is a real pain when the deck and underlayment is put down first and then there is a leak that damages the underlayment that is under the wall.
Frame the bathroom first. You wouldn't want to build the bathroom walls over the flooring (or try to end the flooring where the framing will be).
Framing, then ceiling, then floors. Simply because you don't want new flooring in a construction zone. Especially if you're putting sheetrock on the ceiling.